In recent years, from the viewpoint of global environmental issues, it has become imperative to consider environmental issue, such as energy saving, resource saving and reduction of environmentally hazardous substances, when manufacturing industrial products. Most of the components for machine structural use, such as gears, shafts and nuts, which are manufactured using steel for machine structural use, are manufactured by cutting steel for machine structural use. Therefore, in the field of a cutting process of steel for machine structural use, it is necessary to study cutting techniques and techniques considering environmental issue in terms of steel for machine structural use which is the subject to be cut.
In cutting steel for machine structural use, in order to improve the cutting process efficiency more than in a dry process or a semi-dry process, a process in which cutting oil is used is widely carried out. The cutting oil is classified into water-insoluble cutting oil, which is used as an undiluted solution, and water-soluble cutting oil, which is used after being diluted using water. While both types of the cutting oil are widely used in the cutting of steel for machine structural use, the water-insoluble cutting oil accounts for the majority of cutting oils used in a case in which a process in which finish accuracy is required, such as gun drilling, gear cutting, broaching or tapping, or a process which is carried out with a large tool load. Therefore, it is necessary to increase the efficiency of a process in which water-insoluble cutting oil is used.
A chlorine-based extreme-pressure agent, which is cheap and has excellent lubrication property, is added to water-insoluble cutting oil; however, due to recent environmental issues, the costs for treating cutting oil including the chlorine-based extreme-pressure agent have increased, and therefore, currently, a sulfur-based extreme-pressure agent is mainly used as an alternative of the chlorine-based extreme-pressure agent. However, since the use of a sulfur-based extreme-pressure agent cannot provide sufficient lubrication property, there is a demand for cutting techniques which can provide excellent lubrication property and increase cutting efficiency.
Meanwhile, regarding the steel for machine structural use, in order to increase the efficiency of a cutting process, it is necessary to increase the machinability of steel, that is, to make steel more easily cut. In the past, S or Pb was added in order to improve the machinability of steel for machine structural use. However, when the amount of S added increases, there is a problem in that the mechanical properties deteriorate. On the other hand, since Pb degrades the mechanical properties only slightly so as to improve the machinability, Pb has been usefully used to improve the machinability of steel for machine structural use. However, there is a problem that Pb is an environmentally hazardous substances. Therefore, there is a demand for improving machinability without using S or Pb.
With the above background, regarding cutting techniques of steel for machine structural use, techniques regarding environment-responsive cutting oil are studied in, for example, Patent Document 1. When an oiliness improver is added to a lubricant made of lubricant-based oil, a dispersant and microparticle-form calcium carbonate, favorable lubrication property can be obtained without including a chlorine-based extreme-pressure additive.
In addition, regarding steel for machine structural use, steel having a new component composition or structure has been studied. For example, Patent Document 2 discloses that, when the amount of solute N, which is harmful to machinability, is suppressed to a low level by adjusting the added amount of high Al, other nitride-generating elements and N, and by carrying out an appropriate thermal treatment, and when an appropriate amount of solute Al which improves machinability through high-temperature embrittlement and of AlN which produces a high-temperature embrittlement effect and improves machinability through a cleavage crystal structure, is secured, steel for machine structural use having excellent machinability throughout a wide cutting rate range from low speed to high speed and having both high impact value and a high yield ratio is obtained.
As described above, regarding environment-responsive techniques in cutting processes, studies are being made on cutting oils and steel components. In the future, when further studying techniques considering environmental issue, it is necessary to study both cutting oils and steel components.